Abstract: A detecting and alarming device is provided, that has a mainframe and a detecting set detachably coupled to the mainframe. The detecting set has a parameter calibrating and setting unit for setting a predetermined concentration parameter, in order for the detecting set to send a signal to an alarm module of the mainframe, when a concentration of an ambient object detected by a detecting element of the detecting set is equal to or exceeds the predetermined concentration parameter. It allows the alarm module to generate an alarming signal upon receipt of the signal transmitted from the detecting set and the detecting, set to be easily separated from/coupled to the mainframe for replacement purpose.